Why Are Asexual Animals Rare?

Dec.13,2010

The Amazon Molly reproduces without any genetic contribution from a male (click for credit)

Most biology students know that there are two forms of reproduction: sexual and asexual. In sexual reproduction, the genome of the offspring is made from two contributors: the male parent and the female parent. In asexual reproduction, a single organism reproduces, and the offspring are genetically identical (barring any mutations) to the single parent. In general, bacteria reproduce asexually, while most animals and plants reproduce sexually. However, there are some animals that can reproduce asexually. Sea stars, hydra, and planarians are examples.

What most biology students don’t know is that there are examples of individual species of animals that reproduce asexually, even though other very similar animals reproduce sexually. Take, for example, the Amazon molly (Poecilia formosa), which is pictured above. It is thought that this species arose when a female Atlantic molly (P. mexicana) sexually reproduced with a male sailfin molly, (P. latipinna). While both the mother and the father (as well as all other members of the genus) reproduce sexually, the Amazon molly reproduces asexually. So when this interesting fish produces offspring, they are all genetically identical to the parent, except in certain rare instances, such as when mutations occur.

Now interestingly enough, there are a few forms of asexual reproduction in animals, and the one employed by the Amazon molly is called “gynogenesis.” In this form of asexual reproduction, a male is needed, but he contributes nothing to the genetics of the offspring. Essentially, the female produces eggs that have the full complement of genes (technically called a “diploid egg”), but they cannot begin development into offspring until they are stimulated by the presence of a male’s sperm. The problem, of course, is that all Amazon mollys are female. As a result, the Amazon molly “mates” with similar fishes, usually ones from the same genus.

One really interesting question related to all this is, “Why is it rare?” After all, sexual reproduction is annoying. You have to find a member of your own species that is the opposite gender. The Amazon molly’s form of asexual reproduction still requires a male, but it can be from a wide range of species. As a result, it is much easier for the Amazon molly to find a mate. Why, then, isn’t this kind of reproduction found very often in animals?

The common evolutionary answer is that sexual reproduction allows for a wide range of genetic diversity. As a result, it makes a population much more likely to be able to adapt to changing conditions. When all your offspring are clones, you are counting on mutations to provide the genetic diversity, and that’s doesn’t happen very often. As a result, asexual animals are thought to be “evolutionary dead ends” that are at a severe disadvantage compared to animals that sexually reproduce.

Stöck and colleagues wanted to test this explanation, and not surprisingly, they found it wanting. 1 First, they tried to do what a lot of others have tried to do. They tried to make their own version of the Amazon molly by inducing Atlantic mollys and sailfin mollys to mate. After all, that’s how it is thought the Amazon molly arose. However, like other researchers, they just couldn’t produce any Amazon mollys.

So their next step was to examine the mitochondrial DNA of many different Amazon mollys to see if they could get an idea of their origin. Well, they found a lot of genetic diversity. That’s a bit surprising, since the vast majority of offspring are clones of the parent. However, if you assume the Amazon molly has been around for a long time, you can understand it. After all, the longer the Amazon molly has been around, the more mutations and other rare events can lead to genetic diversity, even in an asexual species.

Now, of course, this should be surprising to an evolutionist. After all, according to evolution, asexual reproduction in animals like fishes is so rare because it reduces genetic variability, making the population less able to adapt. Thus, they are not expected to survive for a long time. However, the Amazon molly has been around long enough to produce a lot of genetic diversity. If that’s the case, then, the evolutionary explanation for the rarity of asexual reproduction in fishes is (not surprisingly) wrong. Indeed, a commentary on the article that appears in the same issue of the journal says it

…suggests that vertebrate gynogens such as the Amazon molly are not rare because they are at a disadvantage to their sexual counterparts, but because the genomic conditions under which they arise are rare.2

Now while all this is very fascinating, it’s not the reason I decided to blog about the Stöck paper. Instead, I want mention what it tells me about how the Creator designed His world. I have already discussed how genetics seems to indicate that the Creator designed the ability to adapt right into His creation. This study indicates to me that the designed adaptability is rather far-reaching. Under the right circumstances, it can even produce a change in the way that an organism reproduces!

However, as is the case in any well-designed system, an animal shouldn’t take extraordinary measures if they aren’t needed. In other words, if a well-designed system is going to change, the amount of change should be reasonable given the situation. Small stresses should produce small changes, while large stresses should produce large changes. A change in reproductive strategy would, of course, be a large change, so it should be brought on by a large stress. The fact that we think we know how the Amazon molly formed and yet cannot make it happen ourselves indicates that the Amazon molly might, indeed, have been formed as a result of a large stress.

The more we learn about how organisms adapt over time, the more impressed I become with the design behind their ability to do so! Interestingly enough, the title of the commentary I quoted above is, “Frozen F1’s amidst a masterpiece of nature: new insights into the rare hybrid origin of gynogenesis in the Amazon molly (Poecilia formosa).”

Nature is, indeed, a masterpiece, because it was authored by the ultimate Master!